NBC Bay Area has learned that authorities investigating the deadly Camp Fire have tied its origin to the failure of a single steel hook that held up a high voltage line on a nearly 100-year-old PG&E transmission tower.

The fire began at the base of a transposition tower, which serves to redistribute the electricity on the system to balance the load and assure safety. The tower has two arms holding out the “jumper,” a part of the line that’s being shifted to another point at the top of the tower.

The arms each hold electrical insulators, which resemble a series of white discs. Authorities believe the fire started with the fracturing of a steel hook that holds up the insulators to the arms above. It is one of those hooks, sources with knowledge of the investigation say, that failed in high winds the morning of Nov. 8.

“PG&E failed to maintain the tower, and they have an obligation to do that — and it means they are liable for this disaster,’’ said attorney Dario de Ghetaldi, who is suing the utility over the fire.

Cal Fire would not comment for this story, citing the ongoing investigation. PG&E would not address our information on the apparent cause, instead sending us a statement saying, “The cause of the Camp Fire is still under investigation. We continue to focus on assessing infrastructure, safely restoring power where possible and helping our customers recover and rebuild.”

But Frank Pitre, another attorney who has sued over both wildfires and the San Bruno gas explosion, worries the hook failure is just more evidence that PG&E is simply not able to deal with the risk posed by its system.

“That’s a red flag,” he said, a warning that parts of the system could fail due to corrosion and fatigue from decades of service.

“You have to a have a rigorous system of inspection, particularly when you have systems that are 50 years old or more. You can’t just visually inspect these things.’’

De Ghetaldi says PG&E should have done those detailed inspections after 2012, when five towers collapsed during a winter wind storm. “They should have taken a serious look at the entire circuit,” he said, “after those five towers collapsed in 2012, and it appears they didn’t do that. That’s a big problem for PG&E. And it’s a big problem for the people who were killed, who were made homeless, and who were harmed in many ways.”